The present invention relates to planar light source devices used for a backlight unit for a liquid crystal panel, in particular to a planar light source device employing approximately U-shaped cold cathode fluorescent tubes.
Various backlight units employing a cold cathode fluorescent tube have been invented to be used for liquid crystal modules such as liquid crystal televisions.
JP 2005-85689 A discloses a planar light source device of the direct backlight type with approximately U-shaped cold cathode fluorescent tubes arranged parallel to an emission plane. The planar light source device disclosed by JP 2005-85689 A includes the approximately U-shaped lamps in which each of the U-shaped lamps is arranged so that the projection side is positioned parallel to the emission plane.
JP 2004-71576 A discloses a planar light source device provided with a straight cold cathode fluorescent tube oriented along each of widthwise edges of the enclosure. The planar light source device disclosed by JP 2004-71576 A is provided with a backside translucent film for guiding light emitted from the straight cold cathode fluorescent tubes to an emission plane. The backside translucent film is arranged so that the distance between the film and the emission plane gradually decreases as the film stretches from the edges toward the center of the enclosure lengthwise.
JP 2005-195778 A discloses a planar light source device of the direct backlight type provided with straight cold cathode fluorescent tubes arranged parallel to an emission plane. The planar light source device disclosed by JP 2005-195778 A includes cold cathode fluorescent tubes and a chassis. The distance between the tubes and the chassis is greater around the edges of the tubes to which a voltage is applied than around the opposite edges of the tubes.
JP 2005-150056 A discloses a planar light source device of the direct backlight type provided with straight cold cathode fluorescent tubes arranged parallel to an emission plane. The planar light source device disclosed by JP 2005-150056 A includes cold cathode fluorescent tubes which are arranged so that the tubes disposed inside widthwise are positioned closer to the enclosure side, i. e., the side of a reflection plane, than the tubes disposed outside widthwise of the enclosure. The cold cathode fluorescent tubes of JP 2005-150056 A are further arranged so that the distance between the tubes disposed inside widthwise is shorter than the distance between the tubes disposed outside widthwise.
Each of the planar light source devices described above is arranged so as to have a uniform distribution of an irradiation amount on the emission plane. In the meantime, some liquid crystal television devices require higher brightness around the center of the display than around the edges for an improved user-visibility. However, the conventional planar light source devices employing direct-type cold cathode fluorescent tubes have not been able to attain higher brightness around the center of the display widthwise (i. e., in the shorter-side direction) and lengthwise (i. e., in the longer-side direction) at the same time.
The present invention aims to provide a planar light source device of the direct backlight type which achieves a higher brightness around the center of the display than around the edges.